1. Field of the Invention
The present invention relates to an image forming apparatus and, in particular, to a method of controlling temperature of a fuser of an image forming apparatus, in which the temperature of the fuser is measured, a temperature ascending stage and a temperature descending stage are differentiated from one another, and optimized temperature control is performed for each stage.
2. Description of the Related Art
An ordinary image forming apparatus, such as an electrophotographic image forming apparatus prints out a desired image by developing an electrostatic latent image formed on a photosensitive medium using toner to form a toner image, transferring the toner image on the photosensitive medium onto a sheet, and fixing the toner image on the sheet by applying heat and pressure in the fuser.
FIG. 1 is a perspective view schematically showing a conventional fuser. The fuser 10 comprises a fixing roller 11, and a compression roller 12 installed to be in contact with and to be compressed against the fixing roller with a predetermined pressure. A heating lamp 13 is installed within the fixing roller 11 to heat the fixing roller 11. The fixing roller 11 is also provided with a temperature detection sensor 14, so that the temperature of the fixing roller 11 can be detected.
The temperature of the fixing roller 11 of the fuser 10 is set to be varied depending on the operating condition of the fuser 10. Typically, the temperature is set to about 150° C. in the ready mode and set to about 180° C. in the printing mode. The temperature of the fixing roller 11 is controlled by a temperature control unit (not shown in the drawing) that intermittently connects a power supply with the heating lamp 13 in response to an output of the temperature detection sensor 14 that detects the temperature of the fixing roller 11.
A conventional method to control the temperature of a fuser is performed by turning a given switching unit off and on with a predetermined control period. The method divides the temperature range of the fuser, measured through the temperature detection sensor, into several intervals and controls a chopping rate of the switching unit of each interval, as shown in FIG. 2. Chopping rates of the switching unit are shown in Table 1.
TABLE 1IntervalChopping rate (%)T1~T250T2~T330above T30T3~T230T2~T150below T1100
Each chopping rate indicated in Table 1 is defined in such a manner that if an ON signal of power is applied to the heating lamp 13 of the fuser ten times for 100 ms, the chopping rate is defined as 100%. Therefore, 50% means that the ON signal is applied to the heating lamp 13 five times for 100 ms, 30% means that the ON signal is applied to the heating lamp 13 three times for 100 ms, and 0% means that only an OFF signal is applied to the heating lamp 13.
However, if power is applied to the heating lamp by the operation controlled as described above, when an inrush of current having a magnitude of tens of amperes, which is produced due to a sharp drop of AC voltage, is applied, occurrence of overshoot may not be avoided. In addition, the flicker phenomenon that causes the heating lamp 13 to flicker is produced since the power applied to the heating lamp 13 is caused to fluctuate. If the flicker phenomenon is produced, the life span of the heating lamp is shortened. Additionally, precisely controlling the temperature is, image quality of image may be adversely affected.